Electroluminescent (EL) lamps may be divided generally into two types: (1) thin-film EL lamps that are made by depositing alternating films of a phosphor and dielectric material on a rigid glass substrate usually by a vapor deposition technique such as CVD or sputtering; and (2) thick-film EL lamps which are made with particulate materials that are dispersed in resins and coated in alternating layers on sheets of plastic. In the latter case, the thick-film electroluminescent lamps may be constructed as thin, flexible lighting devices thereby making them suitable for a greater range of applications.
A cross-sectional illustration of a conventional thick-film EL lamp is shown in FIG. 1. The lamp 2 has two dielectric layers 20 and 22. A first conductive material 4, such as graphite, coated on a plastic film 12b forms a first electrode of the lamp 2 (this electrode could also comprise a metal foil); while a thin layer of a transparent conductive material 6, such as indium tin oxide, coated on a second plastic film 12a forms a second electrode. Sandwiched between the two conductive electrodes 4 and 6 are two layers 20 and 22 of dielectric material 14 which may be, for example, cyanoethyl cellulose, cyanoethyl starch, poly-(methylmethacrylate/ethyl acrylate) and/or a fluorocarbon polymer. Adjacent to the first electrode 4 is a layer of dielectric material 14 in which are embedded particles of a ferroelectric material 10, preferably barium titanate. Adjacent to the second electrode 6 is a layer of dielectric material 14 in which are embedded particles of an electroluminescent phosphor 8. When an alternating voltage is applied to the electrodes, visible light is emitted from the phosphor.
The electroluminescent phosphors available for thick-film EL lamps are primarily comprised of zinc sulfide that has been doped with various activators, e.g., Cu, Au, Ag, Mn, Br, I, and Cl. Examples of zinc sulfide-based EL phosphors are described in U.S. Pat. Nos. 5,009,808, 5,702,643, 6,090,311, and 5,643,496. Preferred EL phosphors include ZnS:Cu phosphors which may be co-doped with Cl and/or Mn.
The brightness of electroluminescent phosphors, and in particular the ZnS:Cu phosphors, significantly deteriorates due to the presence of moisture during the application of the electric field. It has been reported that the deterioration of the brightness of the zinc sulfide-based phosphors is caused by increasing sulfur vacancy, which is produced by the following reaction:ZnS+2H2O→SO2+Zn+2H2 
Sulfur escapes from the phosphor in the form of SO2; as a result, sulfur vacancy and zinc are left in the phosphor.
Therefore, it is important to incorporate moisture protection measures to prolong the light emission of EL lamps. Typically, the individual particles of EL phosphors are encapsulated with an inorganic coating in order improve their resistance to moisture-induced degradation. Examples of such coatings are described in U.S. Pat. Nos. 5,220,243, 5,244,750, 6,309,700, and 6,064,150. These inorganic coatings are formed via a chemical vapor deposition (CVD) reaction while the phosphor particles are suspended within a gas-fluidized bed. In general, a thin yet continuous coating is deposited upon the surface of the phosphor particles, thereby protecting them from the effects of atmospheric moisture.
A preferred coating for EL phosphors results from the hydrolysis of trimethylaluminum (TMA). The hydrolyzed TMA coating and CVD process are described in U.S. Pat. Nos. 5,080,928 and 5,220,243 which are incorporated herein by reference. The composition of the hydrolyzed TMA coating is believed to be primarily aluminum oxyhydroxide (AlOOH), but may be varied in composition between aluminum oxide and aluminum hydroxide depending upon the reaction conditions. For the sake of convenience, the composition of the hydrolyzed TMA coating will be referred to herein as aluminum oxyhydroxide (AlOOH) although it is to be understood that this also encompasses the full range of compositions from aluminum oxide (Al2O3) to aluminum hydroxide (Al(OH)3). The reaction of TMA and water can be described as follows:Al(CH3)3+(3+n)/2H2O→AlO(3-n)/2(OH)n+3CH4 (0≦n≦3)
FIG. 2 is a graph of the 100-hour maintenance as a function of aluminum content (coating thickness) for conventional AlOOH CVD-coated EL phosphors operated in an EL lamp at 50° C., 90% rel. humidity. As can be seen, the coating thickness corresponding to about 3.8 wt. % aluminum (as a percentage of the total coated phosphor weight) represents in the case of the conventional CVD method the coating thickness for the optimal combination of retained initial brightness and high moisture resistance. As used herein, the 100-hour maintenance is defined as the 100-hour light output divided by 0-hour light output and multiplied by 100%, (100-hour/0-hour)×100%. Compared to the uncoated phosphor, the CVD-encapsulated EL phosphor always suffers a significant loss in initial brightness as a result of the coating process. It is suspected that the decrease may be caused by a decrease in the electric field inside the phosphor particles due to the presence of the outer coating.